Sections include: exploring the senses, the senses' specialties, differences, disorders
The purpose of the major senses is to detect and discriminate among signals coming from our environment. These signals carry information necessary for us to support our vital functions, such as taste and smell in eating, as well as functions used in communicating with others and in our work, such as sight, touch, and hearing. In addition to the traditional five senses, other senses of which we are not aware are at work within our bodies, such as the sense of balance and the sense of muscle effort, called kinesthesia, and many senses involved in detecting chemical changes in the blood and other tissues.
All of these senses are present at birth in the human. Research on newborn babies has shown that when they are tested with different taste solutions before any exposure to feeding, they show the appropriate facial responses, such as smiling at a sweet taste and grimacing at a bitter taste. Since the higher brain centers, in the neocortex, of a newborn are not yet functional, these experiments have shown that our basic emotional expressions of pleasure and pain are hard-wired into our brain stem circuits from birth. Although sight and hearing appear to be rudimentary, careful testing has shown that babies of only a few months recognize their mothers by sight and sound.
Exploring the Senses
Sensory systems have been the subject of much modern research in neuroscience because they are accessible to testing and because one begins by knowing exactly what type of information is processed by them. In contrast, in many central brain systems, it is difficult to pin down what kind of information is being processed.
Each of the different senses has particular sense cells within its particular organs: for sight, photoreceptor cells within the retina at the back of the eye; for hearing, hair cells within the inner ear; for smell, olfactory sensory neurons within the olfactory epithelium at the top of the nasal cavity; for taste, taste cells within taste buds in the tongue and back of the mouth. For touch, there are many different types of receptors in the bare nerve endings in the skin that extend from nerve cells in spinal and brain stem ganglia: for pain, for temperature (heat and cold). In addition, there are specialized receptors in the skin and deep tissues for pressure and for light touch.
We move through the rough-and-tumble physical world with such ease that it is astonishing to realize the exquisite refinement of each of our sensory systems. Several aspects of sensory systems have been especially studied. One of the most important we are trying to understand is the mechanisms by which the signals from the external world are converted into nerve signals. That is, how can a passing molecule of diesel fuel, for example, start the series of brain cell firings that result in our holding our nose? This process is called sensory transduction. One of the main principles emerging is that transduction begins with the sensory signal acting on a protein that sits in a sensitive part of the membrane of the sensory cell. For example, in the eye this protein is the photopigment rhodopsin, which is concentrated in membrane disks within the photoreceptor cells at the back of the retina. It is the first to receive that flash of light from a piece of paper as it flutters to the ground. In the nose, it is a receptor protein that is concentrated in fine hairs that extend from the ends of the sensory cells situated in a patch at the top of the nasal cavity. Research has shown that the sensory protein in the nose belongs to the same family of molecules as rhodopsin. Each protein is adapted to receive its particular sensory signal. They are called G protein-coupled receptors, because a molecule called a G protein (for guanosine triphosphate) must be coupled to them to continue to transmit a signal. When light or an odor activates these receptors, they in turn activate their G proteins.
Researchers have found that activation of a G protein then leads to the production of a small messenger molecule (cyclic adenosine monophosphate [cAMP] or cyclic guanosine monophosphate [cGMP]) that acts on a membrane protein to set up an electrical response in the membrane. Cyclic AMP and cyclic GMP are widespread throughout the body. They are called second messengers because they take the response to the first messenger (the initial signal from outside the cell), amplify it within the cell, and direct their response to an appropriate site within the cell. In the case of sensory cells, this is the electrical response, which in turn generates a discharge of impulses that encodes the strength of the sensory stimulation. Most sensory cells are set at near their physical limits for detecting very weak signals; for instance, the inner ear is set to detect a movement of the tympanic membrane (eardrum) of the width of a hydrogen atom, and the eye is set to detect single photons from starlight on a dark night.
The Senses’ Specialties
It is important that sensory systems not only detect weak signals and determine the strength of a signal but also discriminate between different signals. In vision this includes the discrimination of fine details (called visual acuity) or different wavelengths of light (color discrimination); in smell it involves distinguishing between different smells; in taste it involves distinguishing among the basic tastes of sweet, salt, sour, and bitter; in touch it involves sensing the ways that different objects feel. Discrimination thus requires populations of sensory receptor cells that can respond to different aspects of the stimuli.
All sensory systems provide for such differentlysensitive receptor cells, which give rise to parallel pathways that carry the information to the higher centers where discrimination takes place. These pathways are gathered into nerve tracts that ascend through the lower parts of the brain to the highest centers. Thus, the optic nerves carry information in a highly ordered manner from the retina to a way station called the lateral geniculate body, in the thalamus, whence it is relayed to the primary visual receptive area of the neocortex. The auditory nerve carries the information from the array of hair cells in the inner ear to the cochlear nucleus in the brain stem, from where there are multiple relays through pathways that rise to the medical geniculate nucleus of the thalamus, from where the information is relayed to the primary auditory receptive area of the neocortex. The sense of touch similarly has its pathway from the spinal cord and brain stem to its thalamic nucleus, for relay to the primary somatosensory receptive area of the neocortex. Smell information is carried in the olfactory nerves to the olfactory bulb, for processing and output to a first cortical station at the base of the brain, for output to the olfactory thalamic nucleus and further relay to the neocortical olfactory area. The taste nerves carry taste information from the tongue and oral cavity to brain stem nuclei for relay to the thalamus andon to the neocortical taste area.
Sensory discrimination generally involves conscious sensory perception. This usually takes place in higher sensory centers within the brain, at the level of the neocortex. The ways in which cortical neurons are able to sort out signals theyreceive allow the conscious individual to recognize differences in how strong a signal is, how one form of taste, visual, or sensory information varies from another taste, image, or touch. (These differentiations are known as discrimination.) Such physiological processes underlie the larger brain functions: perception, consciousness, memory, and other higher functions.
The basic functions of high sensitivity for the detection of weak signals, discrimination of increasing stimulus strength, and discrimination between different qualities of a stimulus are present in all humans. However, there can be significant differences. First of all, there are differences during early life. Although the basic sensitivity of the sensory cells appears to be laid down early, it takes time for the central pathways to mature, and the highest centers mature last. Thus the highest levels of sensory perception are generally not reached until the teens and twenties, having been refined by experience, training, and memory.
As we grow older, there are also differences. Hearing begins to fall off during the 40s and 50s, with loss of the highest frequencies first. There is evidence that this is directly due to damage to hair cells, particularly from excessive exposure to loud noise. Thus, while many middle-aged people may take great pride in the high fidelity of their stereo sets and speakers, most of that high performance is not actually available to them because of highfrequency hearing loss. Smell holds relatively constant until the 60s and then begins a slow decline, which also appears to be true of the sense of taste. Whether the loss is due to damage to the sensory cells or to changes in higher centers is not known. There are also many kinds of individual differences among the normal population. Although there may be general agreement on the major types of smells and colors, there can be significant differences in making finer distinctions, as most of us know from personal experience. Some differences are related to gender. A well-known difference is the generally higher acuity of most women for smell, and the variation in this acuity for many women during the menstrual cycle.
Some loss of function in the senses is just a sign of normal aging, as described earlier. In other instances, decline in function may be a symptom of a more serious problem. Hearing loss may be due to damage to the hair cells in the inner ear. Cochlear implants are effective in restoring serviceable hearing in young children. Loud noises and music, an allergic reaction to medications, immune system disorders, and tumors on the auditory nerve can also cause varying degrees of hearing loss. Problems with vision, especially if they occur suddenly, may be the result of a stroke or transient ischemic attack, which is caused by a blockage (usually cholesterol) of the retinal blood vessels. Viral respiratory infections, tumors, or even a blow to the head that injures the olfactory bulb can cause loss of smell (anosmia) or taste (agensia).
The general rule is that because your senses are so dependable, an observation of loss of function in any of them should dictate a trip to the doctor. The doctor can determine whether the problem is a temporary malfunction due to another
malady or refer you to the appropriate specialist for a more thorough examination.
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